1. Field of the Invention
This invention relates to nozzle plates for devices such as ink jet printers for ejecting liquids in the form of very small droplets, to a method of making such nozzle plates, and to heads for such devices provided with such nozzle plates.
2. Description of Related Art
In an ink jet printer, ink is ejected in the form of droplets through a small diameter nozzle provided in a printhead on to a receiving surface. If the surface of the printhead surrounding the nozzle becomes wetted with ink, however, the droplets tend to be diverted from the correct direction of travel or, in extreme cases, cannot be ejected at all.
To overcome this problem, it has been proposed to provide a nozzle plate comprising a plate provided with one or more nozzle holes and having an ink-repellant layer, usually formed of a fluorinated or silicone compound, coated on the surface of the plate having the nozzle hole outlet(s). The object of the layer is to prevent that surface of the plate being wetted by the ink or at least to reduce the tendency of that surface to be wetted by the ink, so that the time before having to clear or replace the nozzle plate is extended. The plate comprises a plate blank which is generally formed of polysulphone or polyimide or other laser-ablatable material, and after the application of the ink-repellant layer to one face thereof, the nozzle hole is formed by exposing the thus-coated blank to a laser beam preferably an excimer laser beam, of appropriate diameter. The nozzle plate so formed, complete with nozzle hole or holes, is then bonded to the body of the printhead with the or each nozzle hole of the plate aligned with a respective ink channel formed in the body.
A range of low surface energy materials has been proposed for the ink-repellent layer but because of its advantageous combination of low surface energy and resistance to wear, this application is particularly concerned with the use of fluorinated ethylene propylene copolymer (FEP) for this purpose. It is believed that the desirable wear-resistance of this copolymer is due at least in part to its crystallinity, and in this respect it differs substantially from most other fluorine-based compounds that have been proposed because whereas coatings from the latter are readily obtained from solution, eg. as described in EP-A-0,576,007, FEP is insoluble or substantially so in most solvents and therefore has to be applied as a dispersion of polymer particles. FEP coatings thus differ in kind from those derived from solution.
The coating of ink jet printhead nozzle plates with FEP has already been proposed in U.S. Pat. No. 5,646,657 and U.S. Pat. No. 5,653,901. U.S. Pat. No. 5,646,657 proposes including a u.v. absorber m the fluid coating mixture to improve the roundness of the hole formed in the coating layer by the excimer laser. We have found, however, that inclusion of the u.v. absorber can reduce the ink-repellency of the layer. U.S. Pat. No. 5,653,901 proposes heat treating the layer so as to soften and flatten burrs in the layer formed in the nozzle-hole forming process.
U.S. Pat. No. 5,208,604 discloses a method of manufacturing an orifice plate comprising the steps of applying a liquid repellant, curing the coating using UV-ray irradiation and forming orifices by using an excimer laser.
The publications U.S. Pat. No. 5,646,657 and U.S. Pat. No. 5,653,901 both describe forming the nozzle hole in the nozzle plate blank by exposing the back surface of the blank (ie. the uncoated surface) to an excimer laser beam and both recommend an FEP layer thickness of about 1 xcexcm (1000 mn). However, we have found it preferable to form the nozzle hole by exposing the front surface of the plate (ie. the coated surface) of the blank to the laser beam. A reason for this is that the shape and quality of the outlet end of the nozzle hole is important for the correct direction of travel of the ink droplets and by exposing the coated surface of the blank to the laser, it is possible to ensure that the face of the plate in which the outlet is to be formed is in the focal plane of a laser beam focussing system.
With this procedure, however, it will be apparent that the mechanism by which the hole is formed in the FEP layer will be different from that of the procedure in which the laser beam is directed initially on to the back of the blank. In the latter case, the hole in the plate is formed, in effect, by explosion of the laser-ablatable material of the blank that is exposed to the laser beam and the hole is subsequently extended forward through the FEP layer in the direction of the laser beam by vaporisation of the layer as a result of the heat and kinetic energy released by the action of the laser on the material of the blank. In the former case, on the other hand, the direction of the laser beam and the direction of formation of the hole in the FEP layer, which is believed to be by the same mechanism of vaporisation since FEP is itself generally transparent to lasers, are opposed. In any event, we have found that when forming the nozzle hole by directing the laser beam at the coated face of the plate and the coating comprises fused FEP particles, general guidelines for operation where the laser beam is directed at the back (uncoated) face of the blank do not apply; in particular it is not possible to obtain nozzle outlet holes of acceptable quality at the recommended layer thicknesses of about 1 xcexcm, particularly at preferred nozzle sizes of 50 xcexcm and below.
We have now found that when directing the laser beam at the coated face of the plate, the consistent production of nozzle hole outlets of acceptable quality is dependent on the thickness of the FEP layer being within a critical range which is substantially below 1000 nm, especially at the smaller nozzle hole sizes such as 50 xcexcm and below.
Thus, according to the present invention, there is provided a method of forming an ink jet printer nozzle plate, said method comprising
providing a nozzle plate blank comprising laser-ablatable material, said blank having on one face thereof an ink repellent layer comprising fused solid particles of fluorinated ethylene propylene copolymer (FEP), said layer being at least 200 nm but not greater than 600 nm average thickness, and
forming a nozzle hole or holes in said coated blank by exposing the coated face of said blank to a laser beam.
While the process of ablation by excimer laser is to be preferred, the present invention is not intended to be restricted to this type of high energy beam. Radiation from other types of laser sources may be employed as a high energy beam.
In a preferred embodiment, the coated blank is bonded to the printhead prior to forming the nozzle hole or holes, to enable each nozzle hole to be formed in direct alignment with a corresponding channel in the printhead. However, formation of the or each nozzle hole prior to bonding the blank to the printhead is not found to affect the functional quality of the nozzles.
The invention also provides a nozzle plate blank suitable for use in the invention, and comprises laser-ablatable material, said blank having on one face thereof an ink repellent layer comprising fused solid particles of fluorinated ethylene propylene copolymer (FEP), said layer being at least 200 nm but not greater than 600 nm average thickness.
Very good results have been obtained consistently at layer thicknesses in the range of about 200 nm to 300 nm.